[RISCV] Add performMULcombine to perform strength-reduction
The RISC-V backend thus far does not provide strength-reduction, which
causes a long (but not complete) list of 3-instruction patterns listed
to utilize the shift-and-add instruction from Zba and XTHeadBa in
strength-reduction.
This adds the logic to perform strength-reduction through the DAG
combine for ISD::MUL. Initially, we wire this up for XTheadBa only,
until this has had some time to settle and get real-world test
exposure.
The following strength-reductions strategies are currently supported:
- XTheadBa
- C = (n + 1) // th.addsl
- C = (n + 1)k // th.addsl, slli
- C = (n + 1)(m + 1) // th.addsl, th.addsl
- C = (n + 1)(m + 1)k // th.addsl, th.addsl, slli
- C = ((n + 1)m + 1) // th.addsl, th.addsl
- C = ((n + 1)m + 1)k // th.addslm th.addsl, slli
- base ISA
- C being 2 set-bits // slli, slli, add
(possibly slli, th.addsl)
Even though the slli+slli+add sequence would we supported without
XTheadBa, this currently is gated to avoid having to update a large
number of test cases (i.e., anything that has a multiplication with a
constant where only 2 bits are set) in this commit.
With the strength reduction now being performed in performMUL combine,
we drop the (now redundant) patterns from RISCVInstrInfoXTHead.td.
Depends on D143029
Differential Revision: https://reviews.llvm.org/D143394